1909.] Hermaphroditism in Mollusc Crepichila fornicata. 



481 



reason of their environment, being only, as Prof. Conklin provisionally calls 

 them, !: physiological varieties." 



In some cases chains were observed in which the proximal individuals were more or 

 less dwarfed, but as the posterior ends of the shells of dwarfs tend to overgrow the 

 surface to which they are attached, especially if such be a small pebble, the dorsal surface 

 of the shell of the proximal individual offers a larger surface than did the pebble, hence 

 the " B :! individuals in such chains are able to and do grow bigger than the " A : ' 

 individuals. In such chains the largest individual is found about the middle, and is often 

 an o or even a £ . 



Chains dredged up just off the shore at "West Mersea were found to consist of larger 

 and more numerous individuals than those dredged up from " the Main," about 20 miles 

 farther down the coast ; these, however, may be phenomena arising from the usual 

 conditions which determine size, as food supply, temperature of medium, chemical com- 

 position of medium, and so on. 



Sex Phenomena in Allied Species. 



Prof. Conklin has made estimations of the relative average volume of the 

 <J 's and ? 's of several species of Crepidvla, obtaining the following relations : — 



The males of C. plana are y ff the size of the females. 



,, C. adunca „ £ 

 C. convexa „ \ 

 „ C. fornicata „ \ „ „ 



He therefore naturally concludes that " There is then a marked sexual 

 dimorphism in these molluscs, the mature females being generally much 



larger than the males ; the females are sedentary, the males locomotive " 



(2, p. 16). 



In another place (5, p. 441) he further states : " In all species of Crepidvla 



the males are smaller than the females . . . . " And again (5, p. 442) he 



states " That in the case of the other species named {convexa, adunca, 



na.vicelloides, plana) the males are never immovably fixed to one spot . . . , 



their shells also are not distorted so as to fit irregular surfaces as is the case 



with the females. In all cases locomotion is limited to small individuals. The 



young of all species and both sexes crawl about freely and rapidly. In 



C. convexa individuals of both sexes retain this power to a limited extent, 



bvi the large females of adunca, navicelloides, and plana become firmly fixed,, 



v:hereas the males of these species remain small and retain, to a certain extent, 



their power of locomotion* .... In ft plana the shell of the male is more 



nearly round than that of the female, and is usually more sharp-pointed at 



the apex .... [In a] number of individuals the older part of the shell has 



the male characters, while the newer part has those of the female.f " In 



* The italics are mine, 

 t 2, p. 16. 



